Add Chris Sidebottom to CONTRIBUTORS.md

CONTRIBUTORS.md

@@ -23,6 +23,9 @@
   * Optimization on AMD Piledriver
   * Optimization on Intel Haswell
 
+* Chris Sidebottom <chris.sidebottom@arm.com>
+  * Optimizations and other improvements targeting AArch64
+
 ## Previous Developers
 
 * Zaheer Chothia <zaheer.chothia@gmail.com>
@@ -212,4 +215,4 @@ In chronological order:
   * [2022-03] Support RISC-V Vector Intrinisc 1.0 version.
   
 * Pablo Romero <https://github.com/pablorcum>
-  * [2022-08] Fix building from sources for QNX
+  * [2022-08] Fix building from sources for QNX